1. Field of the Invention
This invention relates to thermal printing, and, in particular, to a coating for preventing sticking of thermal printing materials to a thermal printhead of a thermal printer.
2. Discussion of the Prior Art
In thermal printing, images are formed by heating heat-activatable materials in an imagewise manner. Such heating is commonly conducted by means of a thermal printhead, which consists of an array of small, electrically heatable elements, each of which is preferably activated by a computer in a time sequence designed to produce imagewise heating. The most common forms of thermal printing are direct thermal printing and thermal transfer printing. Materials suitable for use in either of these forms of thermal printing will hereinafter be called thermal printing materials.
In one form of direct thermal printing, colorless forms of heat-activatable dyes are incorporated into a polymeric binder borne on a suitable carrier such as a sheet of paper or film. Upon application of heat, the colorless forms of the dyes are converted to their colored forms, so that when heating occurs in an imagewise manner, an image is formed in the dye-containing material. The carrier sheet thereby bears the formed image directly, without transfer of imaging material to other surfaces. In this form of printing, it is preferred that the polymeric binder be in direct contact with the thermal printhead during the printing process. Because commonly used polymeric binders are thermoplastic, there is a tendency for them to soften in the heated areas and stick to the thermal printhead, thereby causing malfunctioning of the printing apparatus and reduction in image quality.
Thermal transfer printing differs from direct thermal printing in that the printing process occurs by heat-activated transfer of image-forming material from a donor to a receptor such that the receptor bears the formed image. Imagewise heating of the material to be transferred from the donor to the receptor is accomplished by a thermal printhead, operated in the manner described previously.
The construction of the donor requires that the image-forming material be carried upon a thin, flexible backing, typically paper or film. The image-forming material may take several forms, such as a meltable colored wax, a diffusing dye, or heat activatable reactants which, when combined with other reactants incorporated into the receptor, form a colored compound. Many of the most suitable backing materials, such as polyethylene terephthalate (PET) film, are thermoplastic, and therefore have a tendency to soften and stick to the printhead during the thermal imaging process, thereby causing poor print quality and malfunctioning of the printing machine. It is, therefore, a fundamental problem in the design of such donor materials to provide a means for preventing such sticking.
Prevention of sticking by selecting materials for backings having softening temperatures higher than those encountered by the donor in the printing process is disclosed in unexamined Japanese Patent Application No. J6 248-093-A, wherein copolymers containing acrylonitrile are proposed. Alternatively, materials that remain non-adhesive even though they may be softened by the heat of the printer are disclosed as anti-stick layers in unexamined Japanese Patent Application No. J8 0210-494-A, wherein polyethylene is proposed as a backing material. Both of these materials suffer from high cost and limited availability. The high softening and melting temperatures of polymers containing acrylonitrile give them great heat resistance, but this heat resistance hinders attempts to form them into film in an economically feasible manner. Polyethylene is more easily processed, due to its relatively low melting point of 137.degree. C., but it requires special treatment to give it the mechanical properties necessary for use as a backing for a donor.
Insertion of an anti-stick layer between the thermal printhead and the surface of the thermal printing material which contacts the thermal printhead can be used to minimize sticking. Materials that exhibit non-adhesive properties are well-known. For example, low surface energy materials, such as fluoropolymers and silicones, may be effective. Alternatively, nonpolymeric materials, such as waxes, fatty acids, and metal stearates, have been found to exhibit anti-stick properties. All of these materials, however, exhibit certain physical and economic disadvantages which make alternative means for preventing sticking of donor backing materials to thermal printheads desirable.
Another major consideration in applying anti-stick layers to donor backings is the method by which such layers are to be applied. Since it is desired that anti-stick materials be applied in very thin layers, the most suitable method of application is to dissolve a small amount of the anti-stick material in a relatively large amount of solvent, and coat the resulting solution onto the surface of the printing material which is nearest to the thermal printhead, after which the solvent is evaporated by conventional drying means, leaving a thin polymeric layer. Use of this method of application requires that the anti-stick polymeric material be soluble in at least one suitable solvent. Many anti-stick materials are not readily soluble in commonly used organic solvents.
Although polymeric silicone materials may be soluble in organic solvents and at the same time may exhibit anti-stick behavior, they are very migratory, i.e., they spontaneously spread along surfaces for long distances, thereby contaminating large areas of the coating facilities, as well as the image-forming material. Further, when the donor is stored in roll form, presently known silicones may migrate from the side of the donor material to which they have been applied to the opposite side of the donor, where they may interfere with the thermal transfer imaging process. Crosslinking or high degrees of polymerization of silicone polymers may be helpful in reducing migration, but because even small amounts of uncrosslinked silicones can have a significant negative effect upon imaging, it is difficult to achieve sufficient crosslinking to completely eliminate the migration problem.
Attempts have been made to utilize polymeric materials that are soluble in commonly used organic solvents as anti-stick layers. In particular, in unexamined Japanese Patent Application No. J6-0204-387-A, the use of styrene-butadiene rubber (SBR) as an anti-stick layer is disclosed. While SBR is known to exhibit anti-stick properties in thermal printing, it is also known to exhibit strong adhesion to itself. This self-adhesion poses severe handling problems, since in production and in use, great care would have to be exerted to prevent any part of the SBR-coated side of the donor from touching any other SBR-coated portion of the material. As is further well-known, other unvulcanized rubber materials also exhibit adhesion to themselves or to other materials. The adhesion properties exhibited by SBR and other elastomeric materials would, therefore, tend to indicate that elastomers are unlikely to be useful in the formulation of anti-stick layers.